Great for background theme and atmosphere. Free, so the price is right! This PDF booklet doesn't give you any actual details for playing any games, though.

Once I read it, I then paid a few bucks for the 'Sky Galleons of Mars' PDF. That ruleset looks great though I have yet to try playing it: essentially sailing ships vs primitive steam-driven gunships - only they are flying in the skies over a Barsoomian-style Mars.